Mega Backdoor Roth Calculator
Find how much after-tax money you can add to a 401(k) and convert to Roth each year.
Runs in your browser. Your files never leave your device. See how to check
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How to do it
- 1Step 1: Open Mega Backdoor Roth Calculator and add your input
Type your numbers or text, add your file, or pick the options you need. Nothing is uploaded: everything stays in your browser.
- 2Step 2: What Mega Backdoor Roth Calculator does
Find how much after-tax money you can add to a 401(k) and convert to Roth each year.
- 3Step 3: Read the result and compare
The result appears at once. Change one value to see how the answer moves, then keep the figure that fits your case.
- 4Step 4: Copy or save it
Copy the result or download it. For an important decision, confirm it with the official source or an adviser.
About this tool
Enter your own 401(k) deferrals, your employer's contributions and your pay. The tool subtracts them from the total annual limit to show the room for after-tax contributions, and projects the Roth balance if you did this every year.
How the room is found
The total limit on all contributions to your 401(k) (your deferrals, the employer's money and after-tax contributions) is $72,000 in 2026 ($70,000 in 2025), or 100% of your pay if that is lower. Catch-up contributions for age 50 and over are extra and do not use this room.
What your plan must allow
The strategy needs a plan that accepts after-tax contributions beyond the deferral limit and lets you convert them to Roth (in-plan Roth conversion or an in-service withdrawal to a Roth IRA). Many plans do not. Convert quickly, because earnings before the conversion are taxable.
The projection
The tool grows the yearly room at the return you choose, with monthly contributions. It assumes that you can contribute the same amount every year.
Worked example
In 2026, $24,500 of your own deferrals and $8,000 from your employer leave $39,500 of room under the $72,000 limit. Contributed every year for 20 years at 7%, that would grow to about $1.71 million of Roth money.
Tips for best results
Check every box before you rely on a result, because one wrong digit changes the whole answer. Use the exact figures from your payslip, bill, certificate or contract, and keep a note of the date you calculated. Rates, thresholds and rules change from time to time, so for tax, fees and legal matters treat the output as an estimate and confirm with the official source named on this page. You can run as many scenarios as you like; nothing is saved or sent, and refreshing the page clears the boxes.
Quick facts
- Category: UK & US
- Works in your browser, no sign-up, no upload.
- Your input stays on your device.
- Available in English.
- Content last reviewed:
Frequently asked questions
Is this the same as a backdoor Roth IRA?
No. It uses the much higher 401(k) limit and does not depend on the IRA income rules.
Does the employer match count against the limit?
Yes, all employer contributions count toward the total.
Is it taxable?
Converted amounts are taxable only to the extent of earnings; the after-tax money itself is not taxed again.
How do I know if my plan allows it?
Ask your plan administrator about after-tax contributions and in-plan Roth conversions.